Graphene is defined as a two-dimensional (2D) nanomaterial consisting of one-atom-thick layer of carbon (C) atoms chemically linked by C—C sp2-bonded. In graphene, the C atoms are densely packed in a planar lattice composed of C6 aromatic rings forming a nanosheet. Graphene exhibits exceptional physical and electronic properties.
However, the diverse potential of graphene have not yet been fully exploited, since the first graphene sheets have been synthesized experimentally in 2004 only obtained through the reduction of graphite.
Carbon-based nanomaterials (e.g. graphite, tubes, graphene or diamond) have at least one dimension at the nanoscale (<100 nm). These materials have attracted great attention due to their unique properties and potential applications in electronics, sensors and energy storage. Note that a planar sheet of graphene is a basis for the origin of carbon nanotubes and graphite. These are, therefore, allotropes of carbon materials, having very different structures and properties.
The International Union of Pure and Applied Chemistry—IUPAC compendium officially defines the state of technology related to graphene: “Previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene ( . . . ) it is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure; the term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed”.
It should be noted, especially, that graphene is at least 100 times stronger than steel.
The intrinsic resistance (σint) of graphene is 130 GPa.
The procedures for obtaining graphene still have some limitations due to the low yield of final product. However, it is a relatively simple process in which, in most cases, the graphene nanosheets are generated from the delamination of precursors (e.g. graphite) with layered structures.
Related studies have led to great interest about 2D nanosheets in addition to the graphene nanosheets, such as nanosheets of transition metal chalcogenides, perovskites, and manganese oxide.
The large-scale production of 2D graphene nanosheets remains a major challenge. A variety of techniques for obtaining graphene nanosheets has been reported, such as: by epitaxial growth; chemical vapor deposition—CVD; reduction or chemical exfoliation of graphite in the liquid phase.
Among these above methods, the reduction and exfoliation of graphite allow large-scale reliably and reproducibly to produce graphene. The rapid development of new materials based on graphene has been widely identified in the literature.
In the content displayed on the state of the art, the use of graphene in the manufacture of a steel pipe, duct or riser is unknown.
The American Petroleum Institute—API provides a standardized steel series of Grade ‘X’ (e.g. X40, X50, X60, X70, X80, X120 and X100) for application in pipes, pipelines and risers systems to transport oil and natural gas.
For example, the term ‘X80’ defines a standard yield strength (YS) of API 5L steel with a value not less than 80 ksi (about 551 MPa). The high quality API X120 steel presents values of YS of not less than 850 MPa. In this case, the tensile strength (TS) of the steel is in the order of about 900 MPa or higher.
A number of national and international standards help define the compositions and specifications of steel tubes existing in the prior art.
In the United States, the American Society for Testing and Materials—ASTM also sets important standards for the composition, dimensions and characteristics of steel pipes, such as ASTM A-53, ASTM A-36, ASTM A-135, ASTM A-178, ASTM A-214, ASTM A-285, ASTM A-387, ASTM A-440, ASTM A-515, ASTM A-516, ASTM A-517, ASTM A-500, ASTM A-633, and ASTM A-656, whose main definitions are adopted by consensus worldwide.
More specifically regarding the steel composition (carbon and alloy elements), the American Iron and Steel Institute—AISI defines significant standardization on the classification of steels, which are adopted worldwide. For example, a 1XXX grade steel is a simple carbon steel, or negligible amount of other elements, which the sequential numbering indicates the carbon content, such as: AISI 1045, steel with 0.45% of carbon; or AISI 1095, steel with 0.95% of carbon. Thus, the following commercial steel products already produced, whose chemical composition is well known and standardized, can be cited: AISI 1010, AISI 1020, AISI 1040, AISI 1080, and AISI 1095.
In Brazil, the Brazilian Technical Standards Association—ABNT (acronym from portuguese, “Associação Brasileira de Normas Técnicas”) defines various rules relating to steel tubes (e.g. carbon steel, electroducts, welded), which stand out: NBR 5580; NBR 5585; NBR 5590; NBR 5595; NBR 5596; NBR 5597; NBR 5599; and NBR 8261.
In Germany, the German Institute for Standardization—DIN (from German abbreviation, “Deutsches Institut für Normung”) defines standards with specifications for carbon steel pipes, such as: DIN 1615; DIN 1626; DIN 1628; DIN 2393; DIN 2394; DIN 2440; DIN 2441; and DIN 2458.
In England, the British Standards Institution—BSI reveals the technical standards relating to steel pipes, among which may include: BS 1387; BS 6363; and BS 1139.
However, to date, as shown in the prior art, the main above references for standardization of steel tubes, which stand out from the specifications of API 5L, ASTM, AISI, ABNT, DIN, or BSI, do not mention any feature or to the existence of graphene-based steel tubes, pipes or risers.
It should be noted that some steel products for use in pipelines, including higher levels of resistance, still lack a practical use, although its development is in early stages.
For a better understanding of the present invention, some concepts help to elucidate the state-of-the-art related to tubes, pipes and risers systems.
More specifically, the follow terminologies are used (at the macroscale):
“Tube” or “pipe” is herein defined as reference to a tubular hollow structure, fitted with a longitudinal hole, characterized by the dimensions and an internal diameter (ID) and/or outside diameter (OD) with default values, and that has characteristics of mechanical properties specified for use in many applications, the tube is the basic unit for the construction of pipelines, which are usually attached to one another by circumferential welding.
“Pipeline” is herein defined as an industrial component, a pipe, flexible or rigid, planned for the transport of liquids and gases, or passage of structures; piping systems may include connections such as “T” or “J” types, angular changes in the direction of the tubes, as well as their diameter; pipe represents a set of pipes, valves, pumps, flow controllers and the like.
“Duct” is here called a tube or a pipe, functionalized for the passage of solids, liquids or gases (e.g. gasoduct, i.e. functionalized pipeline to transport natural gas from one place to another); a duct assembles an industrial equipment formed by several successive tubes, resulting in a line of conduct.
“Riser” is defined here as a tube, a pipe or a pipeline configuration, flexible or rigid, used for the transfer of petroleum, oil and oil products, petrochemicals, natural gas, hydrocarbon, biofuels, water and other fluids; the riser also called “uptake tube” is used to transfer and injection of fluids from the wellhead to the Stationary Production Unit.
“Submarine risers” are made of steel pipes, usually rigid, which connect to a floating wells on the seabed, and transport oil, water, gas or mixtures, applied in offshore systems and can reach deep water systems at distances over 1500 meters, already surpassing the deep of 2500 meters; submarine risers are critical components due to high hydrostatic pressure, launch loads, cyclic loading operation and the proper weight that are submitted.
“Land risers” consist primarily of steel pipes that connect a drive for exploration wells in deep underground, on land, and transport oil, water, gas or mixtures, applied to onshore systems.
“Flowline” is called the configuration of the risers when applied to the transport of liquids and injection.
“Natural gas” is a mixture of highly flammable and odorless gas, most common being methane (CH4), and contains other gases such as ethane (C2H5), propane (CH8) and butane (C4H10) is usually not contaminated with sulfur and so is the cleanest fossil fuel during flaming; after recovery, the propane and butane are removed, and natural gas is converted to liquefied petroleum gas (LPG); LPG is transported in pressurized tanks as a source of special fuel for areas not served by natural gas pipelines.
“API steel” is here defined as a standard metal composition of the iron-carbon (Fe—C) system that includes alloy elements, determined by the American Petroleum Institute (API) for practical use in pipelines to transport oil and gas; the concept of API steel is used as the main reference for the development of the composition of the steel tubes, pipes and risers of this invention.
For the purposes of nomenclature and scope of the present invention, the terms “steel tubes, pipes or risers” include all types of tubular structures (e.g. gasoducts, electroducts), being welded (e.g. obtained by the U-O-E process) or seamless (e.g. obtained by Mannesmann process), varying shapes and sizes, and can be used in any application, since they provide at least the minimum requirements for the use in oil transportation.
“Connection” is a term denoting a piece of steel used for the joining of two structures, in which at least one of the structures is a steel tube.
“Welded connection” is defined here as a heat affected zone (HAZ) of joining a tube to another, including the so-called “composite solder” and the weld metal; a welded steel structure must submit within tolerable limits of defects in solder joints; besides a welded connection being subject to brittle fracture, the base metal can inhibit the propagation of brittle crack.
“Column” is defined here as the structure resulting from welding of tubes, pipes and risers systems until they reach certain length.
“Brittle fracture” is one that occurs at the end of the regime of elastic deformation.
“Ductile fracture” is one that occurs at the end of the regime of plastic deformation.
“Fracture resistance” in steel and steel products is the property of preventing the occurrence of a separation of parts of the material by applying a load; the fracture toughness in steels is directly affected by grain size.
It should be noted that among the many uses of steel risers for transporting oil, its application in ultra deep water is particularly critical, since the risers are exposed to severe environmental conditions in service, such as: the compressive forces, corrosion, extreme variations on the environment temperature (between 50 and −40° C.) and pressure (between 50 and 250 bars).
Several documents published in the state-of-the-art report the development of steel pipes and steel alloys. To date, such products and materials were obtained with different chemical compositions and by various processes, but they differ from the present invention due to the fact that herein it is claimed new steel tubes, pipelines or risers obtained by a unique method of manufacturing involving the addition of graphene sheets, which provides superior properties than those without graphene.
The methods of installation of submarine pipelines have undergone significant changes over the past 20 years. The methods of launching submarine risers systems depend primarily on the type of fabrication of risers and the environment. Depending on the characteristics of the location of installation and method of release chosen, different floating units can be used, such as ships, barges or semi-submersible units. The semi-submersibles have advantage over ships and ferries, for greater stability in rough sea conditions, while ships and ferries are more limited to calm sea conditions.
Concerning its structure, the risers can be flexible or rigid, or even a combination of both types, and constitute a part of the overall cost in the fields of oil exploration. These costs are related to the stages of manufacture, installation and maintenance of such structures.
Rigid risers have a homogeneous wall of rigid material, such as steel or titanium.
Flexible risers have walls formed by combining several layers with different functions, which employ materials such as carbon steel, stainless steel, polymers, and aramid fiber.
In the case of rigid steel risers, the cost of steel itself is mainly related to its size, or more specifically to its thickness (for a given material and defined diameter). The reduction in thickness reduces the cost of steel, so the desired thickness should be the minimum but to provide the necessary resistance to the pipeline. In addition to cost savings, more slender lines are lighter and, in consequence, have greater flexibility in installation.
Both the rigid and flexible risers can be installed featuring a variety of settings. The most common configurations of risers are “free hanging” (catenary free), “steep S”, “lazy S”, “steep wave,” “lazy wave” or “pliant wave.” Systems for steel catenary risers—SCR is the most viable of all the settings usually practiced.
The steel catenary risers exceed the use which may not be possible to use flexible risers (e.g. in terms of temperature, pressure or diameter in adverse conditions).
In the Brazilian scenario, for example, the company Petróleo Brasileiro S.A.—PETROBRAS maintained the use of flexible risers as a traditional solution. However, since the recent discovery of a gigantic source of oil in the pre-salt layer of Brazil, specifically in the area of the Santos Basin in ultra deep water (5000-7000 m), there is a technical limitation to the use of flexible risers. This limitation has stimulated the design modification of existing pipelines. According to F. Nepomuceno (2008), the oil found in this area lies at depths that exceed 5000 meters, under an extensive layer of salt.
In the onshore terminals and offshore submarines terminals, rigid steel risers are indicated as most promising technology for transporting oil and gas over long distances and at sub-zero temperatures. Furthermore, increased resistance and/or thicker materials are required in offshore pipelines, due to increased pressure at great depths.
In the scenario of the Pre-salt, so the biggest challenge is to produce steels with high strength, toughness and good weldability so that it can decrease the thickness of the wall duct, and thus obtain an economy of material and a lower oil cost production.
As part of the natural gas exploration, this is done through a refinement to remove impurities and water vapor, and then transported in pressurized pipelines. The U.S. country has over 300,000 miles of pipelines. The distinctive smell associated with natural gas is due to tiny amounts of sulfur compounds (ethyl mercaptan) added during the refining process, to warn consumers about a gas leak.
In the state-of-the-art, it is agreed that the use of natural gas is growing rapidly. Besides being a source of clean fuel, the transportation of natural gas is cheap and easy, as soon as the operating gas pipelines are in operation. In industrialized countries, natural gas is used mainly for heating, cooking, food and vehicles. It is also used in a process to make ammonia fertilizer. The current estimate of natural gas reserves is about 100 million tons. At current levels of use, this source will be available at last about 100 years. Most of the world reserves of natural gas are located in Eastern Europe and the Middle East.
For added safety in the use of tubes, pipes or risers systems, the structural integrity assessment is made using simplified criteria. This assessment incorporates the mechanisms of plastic collapse and mechanical properties (yield stress and yield strength) of the material.
Structural steel used in riser systems, in particular steel for pressure vessels, exhibit substantial increase in fracture toughness, characterized by the integral J on the initial millimeter stable propagation (ductile extension) of a crack. This crack growth is often accompanied by large increase in the plasticity of the material in relation to a stationary crack. The increase in plastic zone in the crack region with the increasing of load of the structure represents, more generally, the dissipation of the work of external forces in the form of energy of plastic deformation.
Therefore, a structure made of ductile material containing a defect continues to support high levels of charge even after the initiation and propagation of stable fracture.
In addition, recent design philosophies address the operation of structures under plastic regime, since the stable propagation of defects contributes to a redistribution of loads and their detection in subsequent inspection operations and maintenance.
Tubular structures with internal pressurization present relative single characteristics in the plastic zone formed ahead of the crack. High pressure tubes, however, have low plastic constraint because they are thin-walled structures, which do not favor the establishment of plane strain state. In addition, there is the formation of a condition of low triaxial loads resulting from the predominance of the membrane tension (due to the internal pressure).
In this context, it is valuable that the final performance of steel pipe and riser systems be a function of the combination of several parameters, as described by Bai-Bai (2005), among which are:
Diameter of the wall pipe (thickness ratio);
Relation between the type of material x service load tension;
Imperfections of the material;
Welding (longitudinal and circumferential);
Corrosion and resulting reduction in the thickness of the wall pipe;
Cracks and crevices (in the pipe and/or welding);
Local stress concentration;
Additional loads and its amplitude.
Several authors have developed analysis patterns to predict the mechanical properties of steel pipes and riser systems. For example, a normalization method of J-R curves of API 5L X80 steel pipe was developed by Zhu-Leis (2008). The experimental determination of toughness and fracture resistance curves (J-R curves) is particularly important in assessing the integrity of the pipeline.
ASTM E1820 provides practical techniques to determine the toughness, resistance to fracture and cracking through the full J-R curve and the crack tip mouth opening displacement—CTOD of a standard sample of steel pipe.
Note, however, that the crack propagation in steel pipe is not only due from the (internal or external) pressure in a deep-sea submarine environment, but also a consequence of fatigue and structural defects of the metallic component.
For this reason, the development of solutions that improve the chemical composition and resistance to fatigue, as well as minimizing the structural defects, it is especially desirable to enhance the integrity of pipelines and risers systems for the oil transportation.
In general, the manufacture of steel is characterized by making a mixture of compounds and coal, generating the initial coke.
Following the above stage, the mixture is heated, which may contain alloying elements defined by weight (wt %), so that sintering occurs with a fine homogenization and smelting of iron ore when the sinter is obtained. It is well known that part of the sintering process consists of a mixture of compounds (partial mixture+coke+return) and heating in a temperature range from 60 to 1200-1350° C., which process is understood by an evaporation of moisture (˜100° C.) and drying; dehydration of hydroxides (˜150 and 200° C.); combustion and exothermic reactions (between 500 and 700° C.) with the decomposition of carbonates; and a sintering zone (from 900° C.), when several reactions occur in the mixture forming sinter (about 1350° C.), and possible (re)oxidation of oxides.
After sintering, the mixture is subjected to cooling. Throughout this process, the transformation of these compounds in steel occurs through the reduction of carbon by oxygen injection, with minimal contamination by a heat treatment (e.g. using electric arc furnace, or plasma furnace or vacuum). This step is also called refining, when a controlled solidification of the steel is produced, which can be poured into metal molds in the form of ingots (e.g. block format, rectangular or round dowel, pre-shaped, thick or thin plates, or in plates).
The main alloying elements of a steel alloy in Fe—C system are: silicon (Si); aluminum (Al); nitrogen (N); niobium (Nb); manganese (Mn); nickel (Ni); calcium (Ca); titanium (Ti); vanadium (V); molybdenum (Mo); chromium (Cr); copper (Cu); and inevitable impurities.
Reduced levels of alloying elements, i.e. low carbon equivalent (Ceq), are desirable for a good weldability and low cost of the steel. At the same time, these levels must be sufficiently balanced so as to produce a hardening of the material by formation of precipitates.
Regarding the geometry of a steel tube, the cross-section profile is derived from calculations that take into account the service pressure and stress of the tube, reaching the equivalent of 60% of the yield strength of the material at room temperature. There is a relationship between the values of the external diameter of the pipe and wall thickness. These values are generally pre-set for the thickness and diameter of the pipe, properly tabulated and agreed to certain applications by corresponding standards (e.g. API 5L, BS 1387 and DIN 1615).
Setting a desired geometry, the manufacture of steel tubes involves thermomechanical processing by controlled rolling, which enables a refining of the microstructure (e.g. ferrite-martensite duplex or non-polygonal ferrite morphology). In general, the API 5L from X50 to X120 grades used in steels pipelines have a microstructure with average grain diameter between 2.0 and 30 μm, although there are still many controversies in the literature with regard to grain size. The rolling process aims to obtain a certain thickness of the plate with a simultaneous increase in mechanical strength of steel.
In microalloyed steels, the rolling technique produces considerable effects on the microstructure of the steel, such as the formation of cavity during the fracture process. These cavities run parallel to the rolling direction of the original plate and form perpendicular to the direction of mechanical stress. These cavities are also called delaminations or splits, and occur during the fracture process and ductile splits as a result of perpendicular stresses to the direction of the fracture propagation, which cause plastic constraint at the crack tip, i.e. out-of-plane constraint during the slit. This tension acts perpendicular to the principal stress during the failure process, favoring the cleavage of large grains or decoesion fracture in weak interfaces on the metal matrix.
The morphology of the delaminations can vary depending on the load, temperature and stress state active. As the plastic constraint and therefore the tension acting perpendicular to the principal stress is greatest at the center of the steel specimen, suggests that the stress of delamination in the center of the specimen are more severe.
The occurrence of delamination within the material alters the mechanical response of steel, since it modifies the local stress state at the crack tip. The density of multiple delaminations that occur near the crack tip can significantly increase the fracture toughness and resistance to cracking (J-R curve) of pipeline steels in high-toughness API 5L.
When the conformation of the tube is made by rolling process by “cold work”, the deformation is associated with increased inner tension, or stored energy, material and tends to decrease the ductility. The internal stresses may be relieved through various methods of heat treatment or annealing in order to restore ductility. In the manufacture of steel, it is clear that both the chemical composition (alloying elements), as the rolling process, are aspects that influence the final properties of tubes, pipes or steel risers for better quality and service performance.
Note that the rolling process also allows the formation of the seamless tubular structure. An effective technique to control the distribution of surface tension and sub-surface of steel is the process of shot peening (blasting) the surface. In this mechanical surface treatment, a compressive stress is introduced into the metal surface by exposure to a jet shot, at high speed, causing a slight depression that is a surface deformation. Consequently, this process introduces compressive stresses on the surface and subsurface layers in order to delay the nucleation and propagation of fatigue cracks, thus improving the fatigue strength of coated materials. According to Ohji-Niihara (2006), this blast is used to modify the surface layers of materials and improve the strength of metal components.
In the state-of-the-art, it was reported a blasting process on the ceramic surface using tungsten granules. One study was reported about the development of AISI 4340 steel coated with tungsten carbide. Another improvement process of surface treatment was reported by Ko-Yoo (2010) using shots of carbon nanotubes (CNTs) with diameters of about 100 nm, launched at high speed to the material surface to improve their surface properties. Due to the high speed of the granules of small size, the region suffers greater deformation blasted the blasting larger granules.
In another example, the surface of an AISI 1045 steel was blasted with shots under pressure of 0.4 MPa, in a time interval from 10 to 300 s, whose granules are characterized by an average diameter of 80 micrometers, and hardness about 850 HV. The distance between the spray nozzle and the sample was about 100 mm. While the blasting process has been reported in the literature suitable for certain applications, it is necessary to adjust the surface treatment of steel tube with a much smaller granules and promote a high quality finish compatible with the service specification.